Is your tank new? If so then this is all part of the nitrogen cycle. There are some good videos on youtube explaining this (cycling your tank). But this is to be expected. Have you tested for Ammonia and/or Nirite this will help you guage where you stand in the cycle. Basically you have to wait for the correct bacteria to take hold. You can do a few things to help this along such as reduce the level of salt in the water (assuming you dont have anything living in the tank just yet). This gives the bacteria more energy to grow and multiply. Once they have the nitrates will reduce and you can slowly increase the salinity back to normal. You can also add beneficial bacteria. Highly suggest DRTims one and only. If your tank is older, and you properly cycled the tank, then you need to think about how you set up your nutrient export methods. Skimmer? Refugium? ATS? Also figure out where this is coming from (excess bio load?). . (note There are loads of test kits each slightly different, so purple doesnt mean much. Its good you are testing, great job this is a key element to successful reefing, but some numbers wouldn't go amiss that way we can help you more) Hope this helps. Thanks for reaching out Sent from Singapore Reef Club mobile app